Quenched-in shear transformation zones and Schmid-like visco-elastic creep in metallic glass

The absence of periodic atomic arrangement makes it difficult to detect microscopic defects in metallic glasses, yet their roles in macroscopic shape change are prominent. Motivated by probing defects mechanically, we perform stepwise tensile creep at ambient temperature on as-spun metallic-glass ribbons that show structural anisotropy in selected area electron diffraction. The main discovery is a non-linear strain response in which the segmentally added stress causes the creep strain to initially decrease before increasing. Some strain components quickly revert after stress removal when cyclic creep is conducted, but others do not. Only the first cycle exhibits this slower-than-creep reversal; subsequent cycles have reversible strain components. The non-linearity and slow reversal are eliminated after annealing. No measurable relaxation-enthalpy change is detected within the present fast-differential-scanning-calorimetry sensitivity, suggesting that the low-stress response is not dominated by stored rejuvenation. Molecular-dynamics simulation replicates the strain responses under stepwise and cyclic creep, revealing different non-affine responses of atoms at low and high stresses. The observed behavior can be interpreted within a unified framework involving the depletion of a finite quenched-in shear-transformation-zone population together with the increasing contribution of stress-activated shear-transformation-zones. These results support an orientation-dependent, Schmid-like interpretation of low-stress visco-elastic creep in metallic glass ribbons.